1. Field of the Invention
The present invention relates to a cut-off adjusting apparatus for a display using CRT (Cathode Ray Tube), and particularly to a circuit for detecting and correcting cathode current.
2. Description of the Related Art
The cut-off adjustment is generally defined as a critical bias voltage adjustment of a cathode voltage to a first grid voltage of CRT which is a boundary at which a raster is deleted or displayed from a screen. In other words, the cut-off adjustment is defined as such a black level adjustment that the dispersion of a cut-off voltage is absorbed and the black level is kept constant.
The display using CRT has such a problem as variation of cathode current due to time lapse, specifically brightness variation.
As a countermeasure to the above problem has been known a cut-off adjusting apparatus for receiving a specific signal for detecting cathode current to detect the cathode current corresponding to the signal and adjusting the cathode current, thereby performing the cut-off adjustment.
FIG. 4 is a circuit diagram showing the construction of a conventional cut-off adjusting apparatus.
As shown in FIG. 4, the cut-off adjusting circuit 10 comprises a video amplifier 11, a cathode voltage clamp capacitor 12, a cathode voltage clamp resistor 13, a diode 14, a cut-off control amplifier 15 for setting a cut-off voltage of CRT 20, a cathode current detecting signal source 16, a cathode current detecting resistor 17, an analog/digital converter (ADC) 18 and a system controller 19 such as a microcomputer.
In the cut-off adjusting apparatus 10, a video signal SIM is input to the video amplifier 11 to be amplified with a predetermined gain, the DC component thereof is cut in the capacitor 12 and only the alternating component is transmitted to the cathode 21 of CRT 20.
At this time, the DC component for driving the cathode 21 of CRT 20 is reproduced in a DC reproducing circuit formed of the diode 14, etc. and supplied to the cathode.
When the cathode current detection is carried out to correct the cathode current and the cut-off adjustment is carried out, a cathode current detecting reference signal SREF as shown in FIG. 5A is supplied to the video amplifier 11 by the signal source 16 whose driving operation is controlled by the system controller 19. The potential of the cathode 21 at this time is shown in FIG. 5B.
As shown in FIG. 5, the detection of the cathode current is carried out within a video signal section of a horizontal period, and it is shown in FIG. 6 if it is observed at a vertical period.
The cathode current thus detected is converted to a voltage value by the cathode current detecting resistor 17, and then converted to digital data by ADC 18.
In the system controller 19 for controlling the display set, the digital value is compared with the digital value by ADC 18 to detect the error therebetween, and the base potential of a pnp type transistor P15 connected between the cathode of the diode 14 and the ground line is controlled on the basis of a control signal S19 so that the error is nullified.
FIG. 5 representatively shows one of three primary colors R(red), G(green), B(blue). FIG. 5C shows the voltage waveform in the current detecting resistor 17.
FIG. 6 shows a state in which the video signal is omitted. As shown in FIG. 6, only a part of the detection signal appears as shown in FIG. 6B when observed at the vertical period.
However, the cut-off adjusting apparatus described above has the following disadvantages.
1) A cathode current detecting signal source is needed, and thus hardware therefor is needed, resulting in rise-up of the cost.
2) It is practical in a region having a relatively low horizontal deflection period, specifically below 30 kHz and at a specific frequency, however, in a region having a high horizontal deflection period, that is, above 30 kHz and in a system to which the frequency is not specified, an adverse effect such as dynamic focus remarkably appears as shown in FIG. 5C, so that it is difficult to detect the cathode current.
3) For the reason of 2), the error of the current value detected is large and as a result a large error occurs in the A/D conversion result, so that it is difficult to perform high-precision control. Therefore, it is difficult to perform correction control corresponding to the time variation even when this circuit is used.
4) The current detection is carried out by measuring a specific one point of brightness level. Therefore, when leakage of current occurs in the cathode, an error occurs in the measurement value and it may serve as a control error to induce variation of brightness.
The present invention has been implemented, and an object thereof is to provide a cut-off adjusting apparatus which needs no signal source for detecting cathode current, and can aim to reduce the cost, detect the cathode current with high precision and perform the correction corresponding to the time variation with high precision.
In order to attain the above object, according to the present invention, a cut-off adjusting apparatus for adjusting the cut-off of a display using a cathode ray tube, comprises: video signal processing means which can adjust at least contrast on the basis of a first control signal, and adjust the pedestal level of a video signal to the black level irrespective of input of the video signal and output the signal; a clamp capacitor having a first electrode connected to the output terminal of the video signal processing means and a second electrode connected to a cathode of the cathode ray tube; a rectifying element provided so that a forward direction is set from the connection point between the second electrode of the clamp capacitor and said cathode toward the reference potential; cut-off control means for adjusting the amount of current discharging from the clamp capacitor to the reference potential in accordance with a second control signal to keep the cathode voltage to a predetermined voltage; and current measuring means for outputting the first control signal to the video signal processing means to output a signal of black level when receiving an enforced adjustment instruction, and measuring the current flowing from the clamp capacitor through the rectifying element to the reference potential as a cathode current at the time of the black level signal.
According to the present invention, a cut-off adjusting apparatus for adjusting the cut-off of a display using a cathode ray tube, comprises: video signal processing means which can adjust at least contrast on the basis of a first control signal, and adjust the pedestal level of a video signal to the black level irrespective of input of the video signal and output the signal; a clamp capacitor having a first electrode connected to the output terminal of the video signal processing means and a second electrode connected to a cathode of the cathode ray tube; a rectifying element provided so that a forward direction is set from the connection point between the second electrode of the clamp capacitor and the cathode toward the reference potential; cut-off control means for adjusting the amount of current discharging from the clamp capacitor to the reference potential in accordance with a second control signal to keep the cathode voltage to a predetermined voltage; and current measuring means for outputting the first control signal to the video signal processing means to output a signal of black level when receiving an enforced adjustment instruction, measuring the current flowing from the clamp capacitor through the rectifying element to the reference potential as a cathode current at the time of the black level signal, comparing the current value thus measured with a predetermined set value, and outputting the second control signal to said cut-off control means so that the measured value is equal to the set value.
According to the present invention, a cut-off adjusting apparatus for adjusting the cut-off of a display using a cathode ray tube, comprises: video signal processing means which can adjust contrast on the basis of a first control signal, adjust the pedestal level of a video signal to the black level irrespective of input of the video signal and output the signal, adjust brightness on the basis of a second control signal, and adjust the pedestal level to gray level and output the signal; a clamp capacitor having a first electrode connected to the output terminal of the video signal processing means and a second electrode connected to a cathode of said cathode ray tube; a clamp resistor connected between a predetermined power source and the connection point between the second electrode of the clamp capacitor and the cathode; a rectifying element provided so that a forward direction is set from the connection point between the second electrode of the clamp capacitor and the cathode toward the reference potential; cut-off control means for adjusting the amount of current discharging from the clamp capacitor to the reference potential in accordance with a third control signal to keep the cathode voltage to a predetermined voltage; and current measuring means for outputting the first control signal to the video signal processing means to output a signal of black level when receiving an enforced adjustment instruction, measuring the current flowing from the clamp capacitor through the rectifying element to the reference potential as a cathode current at the time of the black level signal, holding the current value thus measured, outputting the first and second control signals to the video signal processing means to output a signal of gray level, measuring the current flowing from the clamp capacitor through the rectifying element to the reference potential as a cathode current at the time of the gray level signal, and taking the difference between the measured current value at the gray level time and the held measured current value at the black level time.
According to the present invention, a cut-off adjusting apparatus for adjusting the cut-off of a display using a cathode ray tube, comprises: video signal processing means which can adjust contrast on the basis of a first control signal, adjust the pedestal level of a video signal to the black level irrespective of input of the video signal and output the signal, adjust brightness on the basis of a second control signal, and adjust the pedestal level to gray level and output the signal; a clamp capacitor having a first electrode connected to the output terminal of the video signal processing means and a second electrode connected to a cathode of the cathode ray tube; a clamp resistor connected between a predetermined power source and the connection point between the second electrode of the clamp capacitor and the cathode; a rectifying element provided so that a forward direction is set from the connection point between the second electrode of the clamp capacitor and the cathode toward the reference potential; cut-off control means for adjusting the amount of current discharging from the clamp capacitor to the reference potential in accordance with a third control signal to keep the cathode voltage to a predetermined voltage; and current measuring means for outputting the first control signal to the video signal processing means to output a signal of black level when receiving an enforced adjustment instruction, measuring the current flowing from the clamp capacitor through the rectifying element to the reference potential as a cathode current at the time of the black level signal, holding the current value thus measured, outputting the first and second control signals to the video signal processing means to output a signal of gray level, measuring the current flowing from the clamp capacitor through the rectifying element to the reference potential as a cathode current at the time of the gray level signal, taking the difference between the measured current value at the gray level time and the held measured current value at the black level time, comparing the differential value thus taken with a predetermined set value, and outputting the third control signal to the cut-off control means so that the differential value is equal to the set value.
According to the present invention, the current measuring means measures the current while averaging the current at a vertical period.
According to the present invention, the current measuring means performs the measurement of the cathode current averaged at the vertical period at a specific timing of the vertical period.
According to the present invention, upon receiving the enforced adjustment instruction, the current measuring means outputs the first control signal to perform the current measurement when a preset time elapses from the time at which power is turned on.
According to the present invention, horizontal and vertical deflection systems of the display are set to any fixed frequencies when the enforced adjusting instruction is made.
According to the present invention, the set value set by the current measuring means is changeable from the outside.
According to the present invention, the current measuring means generates the third control signal on the basis of the differential value thus obtained and outputs the third control signal to the cut-off control means at other times than the time at which the enforced adjustment instruction is made.
According to the present invention, when the enforced adjusting instruction is made to the current measuring means from the external, the first control signal is output from the current measuring means to the video signal processing means.
At this time, for example, in order to exclude the drift of the power-on, the first control signal is output when a preset time elapses from the power-on time.
In the video signal processing means, the contrast is reduced upon reception of the first control signal, and signal whose pedestal level is black level is output irrespective of input of video signals. This output signal is applied to the cathode of CRT through the clamp capacitor.
The clamp operation is carried out by the action of the clamp capacitor, the rectifying element, etc. At this time, the cathode current is charged into or discharged from the clamp capacitor. The discharge current flowing from the clamp capacitor through the rectifying element to the reference potential is measured as cathode current by the current measuring means.
When the cathode current is measured, the averaged cathode current is measured at a specific timing of a vertical period, for example.
Further, in the current measuring means, the current value measured is compared with a predetermined set value, and a second control signal is output to the cut-off control means so that the measured value is equal to the set value.
According to the present invention, when the enforced adjusting instruction is made to the current measuring means from the external, the first control signal is output from the current measuring means to the video signal processing means.
At this time, for example, in order to exclude the drift of the power-on, the first control signal is output when a preset time elapses from the power-on time.
In the video signal processing means, the contrast is reduced upon reception of the first control signal, and signal whose pedestal level is black level is output irrespective of input of video signals. This output signal is applied to the cathode of CRT through the clamp capacitor.
The clamp operation is carried out by the action of the clamp capacitor, the rectifying element, etc. At this time, the cathode current effected by leak current or the like is charged into or discharged from the clamp capacitor. The discharge current flowing from the clamp capacitor through the rectifying element to the reference potential is measured as cathode current by the current measuring means, and this measured value is temporarily held.
Next, the second control signal as well as the first control signal is output from the current measuring means to the video signal processing means, whereby the brightness adjustment is carried out in the video signal processing means and a signal whose pedestal level is adjusted to gray level is output.
Consequently, the current flowing from the clamp capacitor at the time of the gray level signal through the rectifying element to the reference potential is measured as cathode current by the current measuring means.
The differential value between the measured current value at the gray level time and the measured current value at the black level time held is determined.
That is, in this case, the detection of the cathode current is carried out by a two-point measurement of the cut-off of the cathode current and a current point higher than the cut-off.
In the current measuring means, the differential value thus obtained is compared with a predetermined set value, and a third control signal is output to the cut-off control means so that the differential value is equal to the set value.